1. Field of the Invention
The present invention relates to a light emitting device capable of preventing damage caused by thermal expansion and high temperature which are ascribable to a sealing material, and capable of enhancing light extracting efficiency, and the invention also relates to a method of producing the light emitting device.
The invention also relates to a light emitting device having a phosphor layer on the light emitting side and in which an LED element is sealed with a glass sealing member, and the invention also relates to a method of producing the light emitting device.
Herein, a light emitting element is also referred to as LED element.
2. Description of the Related Art
Japanese Patent Application Laid-open No. 2002-314142 discloses a light emitting device in which a light emitting element is sealed with a sealing material made of silicon resin.
FIG. 15 is a sectional view showing a conventional light emitting device. This light emitting device 100 includes a resin portion 101 having a recess 101A, leads 102 and 103 formed in the resin portion 101 by insert molding, a light emitting element 105 mounted on the lead 102 exposed from a bottom surface of a recess 101A through an adhesive 104 such as a silver (Ag) paste, bonding wires 106 and 107 made of gold (Au) for connecting two electrodes (not shown) of the light emitting element 105 to the leads 102 and 103, a sealing body 109 including a phosphor 108 and charged into the recess 101A, and a reflection film 110 formed on an inner surface of the recess 101A.
The leads 102 and 103 are formed by cutting predetermined portions from a lead frame, and one ends thereof are closely opposed to each other, and the other ends thereof extend in the opposite directions, and pulled out from the resin portion 101.
The recess 101A is of a substantially elliptic shape or circular shape having an inclined surface whose inner diameter is reduced downwardly. The reflection film 110 is formed on the entire region of the inclined surface of the recess 101A using aluminum or the like.
The light emitting element 105 is of a laminated structure having a light emitting layer comprising a nitride semiconductor on a substrate using MOCVD (Metalorganic Chemical Vapor Deposition) or MBE (Molecular Beam Epitaxy).
The sealing body 109 is made of silicon resin. With this, sufficient durability can be obtained even with respect to short wavelength light having peak wavelength of less than 400 nm.
One or more kinds of phosphors 108 can be used. For example, when a plurality of kinds of phosphors are used, there is a combination of three kinds, i.e., a phosphor which emits red light, a phosphor which emits green light and a phosphor which emits blue light. Wavelength conversion can be carried out by the phosphor 110. When the phosphor 110 includes a red phosphor, a green phosphor and a blue phosphor, light such as ultraviolet light emitted from the light emitting element 105 is converted in wavelength by the phosphor 108, secondary light by them is mixed into primary light by the light emitting element 105, and this is extracted as emitting light of the light emitting device 100.
According to the light emitting device 100, since the phosphor 110 is included in the sealing body 109, the phosphor 110 absorbs the primary light by the light emitting element 105 and emits the visible light. Thus, it is possible to obtain a light emitting device 100 whose tone is not changed even if the light emitting wavelength of the light emitting element 105 is changed.
According to the conventional light emitting device, however, when the sealing body 109 is made of silicon resin, the sealing body has excellent weather resistance, light resistance, mechanical durability and the like, but since thermal expansion is generated, the bonding wires 106 and 107 are prone to be deformed at the time of sealing.
One example of a material having small thermal expansion is glass. Since the glass has high viscosity, it is necessary to increase the working temperature, and there is an adverse possibility that the light emitting element is damaged.
Further, if the reflection film 110 in the recess 101A is made of aluminum, since it metallic absorbs light, high reflection efficiency can not be obtained. When the reflection film is made of silver, it is blackened when it is deteriorated, and the reflection efficiency is lowered.
When white resin is used for the resin portion, light-gathering power can not be obtained, and sufficient light extracting efficiency can not be obtained.
In recent years, to eliminate harmful effect with respect to environment (human body) caused by mercury gas, and to reduce the consumed electricity cost, development of a light emitting device using light emitting diode (LED) element has started instead of using cold cathode fluorescent tube in which mercury gas is sealed.
Generally, when white illumination light is to be obtained as a light emitting device of this kind, red, green and blue LED elements, or blue-green and yellow LED elements are used.
There is also used a light emitting device capable of obtaining white light by exciting a phosphor which emits yellow by light emitted from a blue LED element.
Japanese Patent Application Laid-open No. 2001-345482 discloses a light emitting device having a casing including an electrode which opens toward a subject to be illuminated, an LED element accommodated in the casing, a resin mold member for sealing the LED element, and a plastic film including a phosphor for covering the resin mold member.
In this light emitting device, the LED element is an LED element which emits blue light, and the phosphor is excited by blue light and emits yellow light. White light in which emitted light having blue light emitted from the LED element and yellow wavelength conversion light emitted from the phosphor are mixed is emitted toward the light emitting side (toward subject to be illuminated).
According to the light emitting device disclosed in Japanese Patent Application Laid-open No. 2001-345482, however, the resin mold member is deteriorated by heat generated by light emitting of the LED element, the resin mold member is discolored and its transparency is deteriorated, and there is inconvenience that illumination light having high brightness can not be obtained for a long term.